Interleaving-rod type vacuum interrupter having two breaks in series between each pair of rods

ABSTRACT

A vacuum-type electric circuit interrupter comprises interleaving rod electrodes of alternating polarity arranged in a ring pattern and a tubular metal shield disposed around the ring of rod electrodes and normally electrically isolated from the rod electrodes. The tubular shield comprises metal partitions projecting from regions radially outside the ring radially inward into the spaces between immediately-adjacent rod electrodes, thereby dividing each of these spaces into two series-related arcing gaps respectively disposed on opposite sides of each of said metal partitions.

United States Patent 1 Sofianek [4 Jan. 15, 1974 [5 INTERLEAVlNG-RODTYPE VACUUM 3,670,123 6/1972 Luehring 200/144 B INTERRUPTER HAVING TWOBREAKS IN 3,702,911 11/1972 Schonhuber 200/144 B SERIES BETWEEN EACHPAIR OF RODS [75] Inventor: Joseph C. Sofianek, Broomall, Pa.

[73] Assignee: General Electric Company,

Philadelphia, Pa.

[22] Filed: Jan. 19, 1973 [21] App]. No.: 325,181

52 US. Cl 200/144 B [51] Int. Cl. HOlh 33/66 5 8] Field of Search200/144 B [56] References Cited UNITED STATES PATENTS 2,897,322 7/1959Reece 200/144 B 3,321,598 5/1967 Streater.. 200/144 B 3,643,047 2/1972Rich 200/144 B Primary ExaminerRobert S. Macon Att0rney.l. WesleyHaubner et al.

[5 7] ABSTRACT A vacuum-type electric circuit interrupter comprisesinterleaving rod electrodes of alternating polarity arranged in a ringpattern and a tubular metal shield disposed around the ring of rodelectrodes and normally electrically isolated from the rod electrodes.The tubular shield comprises metal partitions projecting from regionsradially outside the ring radially inward into the spaces betweenimmediately-adjacent rod electrodes, thereby dividing each of thesespaces into two series-related arcing gaps respectively disposed onopposite sides of each of said metal partitions.

11 Claims, 3 Drawing Figures BACKGROUND This invention relates to avacuum-type electric circuit interrupter of the type comprisinginterleaving rod electrodes of alternating polarity arranged in a ringpattern, as is described and claimed in U.S. Pat. No. 3,679,474-Rich,assigned to the assignee of the present invention. More particularly,the invention relates to a vacuum interrupter of that type in which twogaps in series are provided between each pair of adjacent rodelectrodes.

The following additional prior art is of interest with respect to thisinvention: US. Pat. Nos. 2,897,322-Reece; 3,185,798-Titus; 3,l85,799-Greenwood et a]; 3,321 ,598-Streater; 3,372,258-Porter; and3,643,047-Rich.

The interrupter of the aforesaid Rich US. Pat. No. 3,679,474 comprisestwo spaced-apart primary electrode assemblies, each comprising a commonbase and a plurality of rod electrodes extending normal to the base. Theelectrode assemblies are positioned so that the rod electrodes of thetwo assemblies form a ring of rod electrodes in which the rod electrodesof one assembly interleave with those of the other assembly. The rodelectrodes about said ring alternate in polarity, and each pair ofimmediately-adjacent rod electrodes are separated by an arcing gaptherebetween. During an interrupting operation, across these arcing gapsthere are established diffuse arcs which can carry relatively highcurrents without the formation of anode spots.

Rich attributes this ability to carry high currents without forminganode spots to the ability of this interrupter configuration to limit toa very low level the body force F that acts on the conduction pathsthrough the arc discharge across each inter-electrode gap. The tendencyto cause the conduction paths between a pair oppositely-poledarc-electrodes to bunch together and develop a high current densitywhich results in formation of an anode spot appears to be directlyrelated to the body force F. By keeping F low, this tendency to bunch iskept correspondingly low.

While prior vacuum interrupters of this interleavingrod type caninterrupt exceptionally high currents, their voltage-handling capacityhas been limited by the fact that only a single gap is present betweeneach pair of adjacent rods. If, following clearance at current zero,

any of these gaps should arc-over in response to a voltage transientapplied between the terminals of the interrupter, a complete circuit isdeveloped between the terminals of the interrupter via the arcing path;and through this complete circuit, high current can flow, resulting inconditions which delay, or even prevent, successful interruption.

If two gaps in series are present between each pair of rods, thelikelihood of an arcing path developing between the rods issubstantially reduced, and the chances for successful interruption aresubstantially improved.

SUMMARY Thus, an object of my invention is to provide aninterleaving-rod type vacuum interrupter having two arcing gaps inseries between each pair of adjacent rods.

Another object is to divide the space between each pair of adjacent rodelectrodes into two series-related gaps by structurally simple means,the v presence of which does not greatly increase the size of theinter-' rupter.

In my interrupter, there is a set of separable contacts located in aposition disposed centrally of the ring of rod electrodes. When theinterrupter is in closed position, current passes therethrough via thesecontacts. Interruption is initiated by separating the contacts to forman arc therebetween. The arcing products developed by the arc arepropogated radially outward into the gaps between the rods, and thiscauses the gaps to ignite and develop low arc-voltage dischargesthereacross to which the interrupter circuit is transferred from the areinitially formed betweenthe contacts.

Another object of my invention is to provide two gaps in series betweeneach pair of adjacent rod electrodes by means of a structurally simplearrangement that readily lends itself to use in an interrupter in whichinterruption is initiated by separable contacts surrounded by the ringof rod electrodes.

In carrying out my invention in one form I provide around the ring ofinterleaving rod electrode members a tubular metal shield that isnormally electrically isolated from both electrode assemblies. Thisshield comprises metal partitions projecting from regions radiallyoutside said ring radially inward into the spaces betweenimmediately-adjacent electrode members of said electrode assemblies,thereby dividing each of said spaces into two series-related arcing gapsrespectively disposed on opposite sides of each of said metalpartitions. These arcing gaps share the voltage applied between adjacentrod electrode members when the recovery voltage builds up between saidelectrode members at current zero at the end of an arcing period.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of theinvention, reference may be had to the following description taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view, partly in section, of a vacuuminterrupter embodying one form of the invention.

FIG. 2 is a sectional view along the line 2-2 of FIG. 1.

FIG. 3 is a sectional view similar to that of FIG. 2 except showing amodification of the interrupter of FIG. 1

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1,there is shown a vacuum interrupter 10 that comprises an upper electrodeassembly l1 and a lower electrode assembly 12 that are joined inhermetically sealed relationship to opposite ends of a cylindricalinsulating casing 13 to form a hermetically sealed envelope 14. Theinterior of this envelope 14 is evacuated to a pressure of 10 torr orlower. Upper electrode assembly 11 comprises a metal base plate, ordisc, 15 and a plurality of downwardly depending electrodes 16 suitablyjoined to the base plate, as by brazing. Lower electrode assembly 12comprises a metal base plate, or disc, 17 and a plurality of upwardlydepending electrode members 18 brazed thereto. Preferably, each of theelectrode members is a cylindrical rod having a smooth external surface,as

is described and claimed in the aforesaid Rich US. Pat. No. 3,679,474.

The rods of each electrode assembly extend parallel to each other, asshown in FIG. 1, and are arranged in a circular pattern,circumferentially spaced from each other about the circle, as shown inFIG. 2. Referring to FIG. 2, the electrode assemblies are positioned sothat the rods of one assembly interleave with the rods of the otherassembly and together form a ring of circumferentially spaced rods, withthe rods alternating in polarity about the ring. The rods 16 and 18 areof a high purity, nonrefractory metal such as suitably degassed copperor steel.

For protecting the insulating casing 13 from arcing products generatedduring an interrupting operation, a metallic shield 23 comprising a bodyportion 19 of tubular form is provided. This shield body 19 surroundsthe ring of electrodes in spaced relationship to the rods and issupported on the insulating casing 13 by means of a flange 20 suitablyjoined to the shield and extending radially outward therefrom. Flange 20is positioned between upper and lower halves of the casing and is joinedto these halves by suitable seals 21. The shield 23 is so located andshaped that a substantially equal capacitance is present between theshield and each of the electrode assemblies when the interrupter isopen, thus maintaining the shield at a potential substantially midwaybetween that of the two electrode assemblies when the interrupter isopen.

For carrying current through the interrupter when the interrupter isclosed, a pair of separable contacts 30 and 32 are provided. Thesecontacts are located generally centrally of the ring formed by the rodelectrode 16, 18. Contact 30 is a stationary contact fixed to the lowerend of a stationary contact rod 34, and contact 32 is a movable contactfixed to the upper end of a movable contact rod 36. The stationarycontact rod 34 extends through upper base plate and is joined thereto bya suitable brazed joint forming a vacuumtight seal between the rod andthe base plate. The movable contact rod 36 extends freely through anopening in the lower base plate 17, and a flexible metal bellows 38provides a seal between base plate 17 and rod 36 that permits the rod tomove freely in a vertical direction with respect to the base plate. Inthe illustrated interrupter, a metal tube 39 is shown surrounding thebellows 38, and this tube is connected by suitable vacuum tight jointsat its opposite ends with the bellows 38 and the base plate 17.

When the interrupter is to be operated to interrupt the circuittherethrough, movable contact rod 36 is driven in a downward directionfrom its position of FIG. 1 by a suitable operator (not shown). Thisseparates contact 32 from contact 30, thereby establishing a gap betweenthe contacts. Establishment of this gap causes an arc to be developedacross the gap; and in this arc vaporizes contact material, whichmaterial is expelled radially outwardly into the spaces between adjacentrod electrodes 16 and 18. No arcs are initially present across thespaces between the electrodes, but when the arcing products enter thesespaces, the dielectric strength of the spaces is drastically reduced,and the spaces are broken down by the voltage then present thereacross.This breakdown of the spaces between the rods results in an arcdischarge between each adjacent pair of opposite polarity rods.

In the interrupter of my invention, the arc discharge space between eachpair of rod electrodes is divided into two separate series-relatedarcing gaps by providing a plurality of metal partitions 40 constitutinga part of shield 23. These partitions 40 are preferably in the form ofmetal plates 41 of planar configuration disposed in radially extendingvertical planes that include the central longitudinal axis of theinterrupter. Each plate 41 is suitably attached at its radially-outeredge to the tubular portion 19 of the shield, as by brazing or welding.Along the radially-inner edge of each plate 41, there is a metal rod 43of circular cross section extending vertically along the length of theplate 41. This rod 43 is suitably joined to its associated plate 41, asby welding or brazing, or being integrally formed therewith and providesthe partition with a rounded inner edge that serves to reduce electricalstress concentration in this region. In a preferred form of theinvention, the partition 40 extends radially inward from the tubularportion 19 of the shield to a reference line located radially inward ofa reference circle 45 along which the radially innermost surfaces of therod electrodes are located. Since the tubular shield portion 19 is atsubstantially mid-potential, as above described, the partitions 40 whichare electrically connected to the tubular portion 19 are also atsubstantially mid-potential.

The metal partitions 40 serve the important purpose of increasing thebreakdown voltage between each adjacent pair of opposite polarity rods16 and 18 when the usual recovery voltage builds up across theinterrupter following a current zero at the end of an arcing period. Ineach of the spaces between a pair of rod electrodes 16 and 18, there aretwo series-related gaps, one between rod 16 and the partition 40 and theother between the partition 40 and the rod 18. Since the partition 40 isat a potential substantially midway between that of the electrodes 16and 18, substantially half the total voltage across the interrupter isapplied to each gap. In a vacuum, the total breakdown voltage of twoseries-related gaps, each subject to half the applied voltage, issubstantially greater than that of a single gap of the same total lengthsubject to the full applied voltage. Hence, the total breakdown voltageacross the two gaps present between each pair of electrodes 16, 18 issubstantially greater than that present when the partition 40 is absent.

Between the distal end of each upwardly-projecting rod electrode 18 andthe adjacent upper end plate 15, there is a space across which aspark-over will establish a continuous electrical path through theinterrupter. In the illustrated embodiment, each of these end spaces isdivided into two series-related end gaps by a metal cap that extendsgenerally horizontally from the tubular portion 19 of the shield to theradially innermost edge of the partition 40 across the top of thecompartment that surrounds rod 18. Thus, this space adjacent the distalend of each rod electrode 18 is also divided into two series-connectedgaps across each of which half the total voltage appears. Thisrelationship results in improved dielectric strength in this region.

Between the lower end plate 17 and the distal end of each of thedownwardly projecting rod electrodes 16, there is a corresponding metalcap 52 that divides this space into two series-related end gaps, eachbearing approximately half the total applied voltage.

For further protecting the insulating casing 13 from arcing productsgenerated during an interrupting operation, I provide a plurality ofauxiliary shields 70, 71, 72, and 73 adjacent the insulating casing 13and surrounding the main shield 23. Each of these auxiliary shields is atubular metal member disposed concentrically of the casing 13 in aposition adjacent a glass-tometal seal in the casing. Auxiliary shields70 and 73 project inwardly of the interrupter from base plates 15 and17, respectively, and are electrically connected to their associatedbase plates. Auxiliary shields 71 and 72 project in opposite directionsfrom central flange and are electrically connected to the flange. Theseauxiliary shields are radially spaced from the insulating casing 13 andfrom the main shield portion 19 and serve to intercept and condense anyarcing products escaping from the region within the main shield portion19 before these arcing products can reach the insulating casing 13. Theauxiliary shields also serve to shape the electric field so as to reduceelectrical stresses on the glass-to-metal seals.

FIG. 3 shows a modified form of the invention in which the partitions40, instead of being planar in configuration, are curved so as to havean inner surface that is generally concentric with respect to thecylindrical surface of the adjacent rod electrode. Each pair of adjacentpartitions is welded together along a vertical seam 62 so as to formabout each rod electrode a generally cylindrical housing or compartmentthat is open at 63 for about 90 of its periphery to the central portionof the interrupter where the contacts 30, 32 are located. An advantageof this design of FIG. 3 is that the electric field between each rodelectrode and the metal housing or compartment surrounding the rodelectrode is generally uniform in view of the generally concentricuniformly-spaced relationship of these parts. This uniformity of theelectric field contributes to a higher breakdown voltage between the rodelectrodes and the surrounding compartments.

In FIG. 3, the tubular portion of the shield that surrounds the entirering of rod electrodes can be thought of as being constituted almostentirely by the partitions 40. Metal caps corresponding to 50 and 52 inthe embodiment of FIG. 1 are also present in the embodiment of FIG. 3,

In both of the illustrated embodiments, it will be noted that the arcsformed across the arcing gaps between the rod electrodes extendgenerally circumferentially of the interrupter. This results in the bodyforce F on each are being limited to a low value, as is explained in theaforesaid Rich patent, thus enabling the arcs to carry relatively highcurrents without developing anode spots and also limiting to a low valuethe arc voltage developed by each arc. The relatively low level of thearc voltage developed by the circumferentiallyextending arcs plays animportant role in enabling these arcs to be established and maintainedin preference to the single arc between the contacts 30, 32. There is astrong tendency for such a single arc to persist in preference tomultiple arcs over an alternate path if arcing via said alternate pathwill result in the development of a higher total arc voltage than isdeveloped by the single are. In my interrupter, the tendency for thesingle arc to persist is greatly reduced because the sum of the arcvoltages developed by the series-related circumferentially-extendingarcs is less than that which would be developed by a single arc burningbetween the contacts 30, 32.

In order to reduce the likelihood that the recovery voltage willestablish a breakdown across the gap between the contacts 30 and 32, Iprovide the interrupter with an unusually long stroke. In one embodimentof the invention, for example, the stroke is of such a length that thefull gap between the contacts 30 and 32 is approximately 2 inches inlength, which is appreciably greater than the total length of the twoarcing gaps between adjacent opposite-polarity rod electrodes 16 and 18.

To further reduce the likelihood that the recovery voltage willestablish a breakdown between contacts 30, 32, I make the gaps betweeneach of the contacts and the radially-inner edges of the partitionsequal to or greater than the length of the arcing gaps between the rodelectrodes and the partitions 40. This discourages breakdowns acrossthese contact-to-shield gaps, the occurrence of which could develop intoan are between the contacts 30 and 32.

While I have shown and described particular embodiments of ourinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention in its broader aspects; and I, therefore, intend herein tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A high-current vacuum circuit interrupter comprising:

a. a hermetically sealed envelope evacuated to a pressure of 10' tort orless,

b. a first primary arc-electrode assembly comprising a first pluralityof spaced-apart substantially parallel rod electrode members havingarcing surfaces disposed within said envelope,

0. a second primary arc-electrode assembly comprising a second pluralityof spaced-apart substantially parallel rod electrode members havingarcing surfaces disposed within said envelope and interleaving inalternate sequence between the spaced electrode members of said firstarc-electrode assembly, said first and second pluralities of electrodemembers being positioned so as to form a ring of electrode memberswithin said envelope with spaces between immediately adjacent electrodemembers, said members alternating in polarity about said ring, e. atubular metallic shield surrounding said ring and electrically isolatedfrom both of said electrode assemblies when no current is passingthrough said interrupter via said electrode members, said shieldcomprising metal partitions projecting from regions radially outsidesaid ring radially inward into the spaces between immediately adjacentelectrode members of said electrode assemblies, thereby dividing each ofsaid spaces into two seriesrelated arcing gaps respectively disposed onopposite sides of each of said metal partitions,

g. and means for causing electric arcs to be established in saidseries-related arcing gaps during a circuit interrupting operation,

h. said arcing gaps being arranged to share the voltage applied betweenadjacent rod electrode members when the recovery voltage builds upbetween i said electrode members at current zero at the end of an arcingperiod.

2. The interrupter of claim 1 in which:

a. the rod electrode members of said first assembly are connected attheir proximal ends to a common metal base toward which the rodelectrode members of said second assembly project,

b. an end space is present between the distal end of each of saidsecond-assembly rod electrode members and said common base,

0. metal caps are provided dividing each of said end spaces into twoseries related end gaps on opposite sides of the associated metal cap,

d. said metal caps each constitute a portion of said shield and areelectrically connected to the portion of said shield surrounding saidring of electrodes.

3. The interrupter of claim 2 in which:

a. said second-assembly rod electrode members at their proximal ends areconnected to a second common metal base toward which said firstassemblyrod electrode members project,

b.'additional end spaces are present between the distal ends of saidfirst-assembly rod electrode members and said second common base,

0. second metal caps are provided dividing each of said additional endspaces into two series-related end gaps on opposite sides of theassociated second p! d. said second metal caps each constitute a portionof said shield and are electrically connected to the portion of saidshield surrounding said ring of electrodes.

4. The interrupter of claim 1 in which said partitions extend radiallyinward past a reference circle that includes the radially innermostsurfaces of said rod electrode members.

5. The interrupter of claim 1 in which the arcing surfaces of each ofsaid rod electrode members are smooth cylindrical surfaces.

6. The interrupter of claim 1 in which said shield, including saidpartitions, is shaped to form about each rod electrode member a metalcompartment having an internal surface substantially uniformally spacedfrom the outer surface of the rod electrode member located therein.

7. The interrupter of claim 5 in which said shield, including saidpartitions, is shaped to form about each rod electrode membera metalcompartment having an internal surface substantially concentric with ansubstantially uniformally spaced from the cylindral surface of the rodelectrode member located therein.

8. The interrupter of claim 1 in which:

a. said means for causing electric arcs to be established comprises apair of separable contacts located in the space surrounded by said ringof rod electrode members,

b. said contacts are engageable to carry current through saidinterrupter and are separable to establish an inter-contact gap acrosswhich an arc is formed,

c. said inter-contact gap freely communicates with the series-relatedarcing gaps on opposite sides of said metal partitions so that arcingproducts from said inter-contact gap are propagated into saidseries-related arcing gaps to cause arc discharges to form across saidseries-related arcing gaps.

9. The interrupter of claim 8 in which when said contacts are fullyseparated, said interrupter has an intercontact gap of a length greaterthan the sum of the length of the two series-related arcing gaps onopposite sides of each of said partitions.

10. The interrupter of claim 8 in which there is a contact-to-shield gapbetween each of said contacts and the radially inner edge of each ofsaid partitions, said contact-to-shield gaps having a length at least asgreat as the length of each of said series-related arcing gaps.

11. The interrupter of claim 1 in which each of said partitions is of agenerally planar form and includes at its radially-inner end an edgethat is enlarged and rounded to reduce electric stress concentrationsthereadjacent.

1. A high-current vacuum circuit interrupter comprising: a. ahermetically sealed envelope evacuated to a pressure of 104 torr orless, b. a first primary arc-electrode assembly comprising a firstplurality of spaced-apart substantially parallel rod electrode membershaving arcing surfaces disposed within said envelope, c. a secondprimary arc-electrode assembly comprisiNg a second plurality ofspaced-apart substantially parallel rod electrode members having arcingsurfaces disposed within said envelope and interleaving in alternatesequence between the spaced electrode members of said firstarc-electrode assembly, d. said first and second pluralities ofelectrode members being positioned so as to form a ring of electrodemembers within said envelope with spaces between immediately adjacentelectrode members, said members alternating in polarity about said ring,e. a tubular metallic shield surrounding said ring and electricallyisolated from both of said electrode assemblies when no current ispassing through said interrupter via said electrode members, f. saidshield comprising metal partitions projecting from regions radiallyoutside said ring radially inward into the spaces between immediatelyadjacent electrode members of said electrode assemblies, therebydividing each of said spaces into two series-related arcing gapsrespectively disposed on opposite sides of each of said metalpartitions, g. and means for causing electric arcs to be established insaid series-related arcing gaps during a circuit interrupting operation,h. said arcing gaps being arranged to share the voltage applied betweenadjacent rod electrode members when the recovery voltage builds upbetween said electrode members at current zero at the end of an arcingperiod.
 2. The interrupter of claim 1 in which: a. the rod electrodemembers of said first assembly are connected at their proximal ends to acommon metal base toward which the rod electrode members of said secondassembly project, b. an end space is present between the distal end ofeach of said second-assembly rod electrode members and said common base,c. metal caps are provided dividing each of said end spaces into twoseries related end gaps on opposite sides of the associated metal cap,d. said metal caps each constitute a portion of said shield and areelectrically connected to the portion of said shield surrounding saidring of electrodes.
 3. The interrupter of claim 2 in which: a. saidsecond-assembly rod electrode members at their proximal ends areconnected to a second common metal base toward which said first-assemblyrod electrode members project, b. additional end spaces are presentbetween the distal ends of said first-assembly rod electrode members andsaid second common base, c. second metal caps are provided dividing eachof said additional end spaces into two series-related end gaps onopposite sides of the associated second cap, d. said second metal capseach constitute a portion of said shield and are electrically connectedto the portion of said shield surrounding said ring of electrodes. 4.The interrupter of claim 1 in which said partitions extend radiallyinward past a reference circle that includes the radially innermostsurfaces of said rod electrode members.
 5. The interrupter of claim 1 inwhich the arcing surfaces of each of said rod electrode members aresmooth cylindrical surfaces.
 6. The interrupter of claim 1 in which saidshield, including said partitions, is shaped to form about each rodelectrode member a metal compartment having an internal surfacesubstantially uniformally spaced from the outer surface of the rodelectrode member located therein.
 7. The interrupter of claim 5 in whichsaid shield, including said partitions, is shaped to form about each rodelectrode member a metal compartment having an internal surfacesubstantially concentric with an substantially uniformally spaced fromthe cylindral surface of the rod electrode member located therein. 8.The interrupter of claim 1 in which: a. said means for causing electricarcs to be established comprises a pair of separable contacts located inthe space surrounded by said ring of rod electrode members, b. saidcontacts are engageable to carrY current through said interrupter andare separable to establish an inter-contact gap across which an arc isformed, c. said inter-contact gap freely communicates with theseries-related arcing gaps on opposite sides of said metal partitions sothat arcing products from said inter-contact gap are propagated intosaid series-related arcing gaps to cause arc discharges to form acrosssaid series-related arcing gaps.
 9. The interrupter of claim 8 in whichwhen said contacts are fully separated, said interrupter has anintercontact gap of a length greater than the sum of the length of thetwo series-related arcing gaps on opposite sides of each of saidpartitions.
 10. The interrupter of claim 8 in which there is acontact-to-shield gap between each of said contacts and the radiallyinner edge of each of said partitions, said contact-to-shield gapshaving a length at least as great as the length of each of saidseries-related arcing gaps.
 11. The interrupter of claim 1 in which eachof said partitions is of a generally planar form and includes at itsradially-inner end an edge that is enlarged and rounded to reduceelectric stress concentrations thereadjacent.